Organic thin-film light-emitting devices are known to be exquisitely sensitive to water vapor and oxygen. Exposure to either of these materials results in the rapid degradation and failure of the devices, reducing or destroying the utility of the flat panel display or flat panel illumination source utilizing these devices. A number of approaches have been developed to seal the organic thin-film devices to eliminate exposure to oxygen and water vapor. The most commonly used approach in commercial application is to apply an adhesive between the device substrate and an impermeable cover. A variety of means, such as UV exposure, may be used to accelerate the curing of the adhesive. A limitation of this approach is that all known commercially suitable curable adhesive materials provide an inadequate barrier to water vapor, which creates the need to introduce a desiccant between the cover and substrate, increasing costs and complexity. Kadowaki, et al. (U.S. Pat. No. 5,693,111) propose to utilize a laser to create a bond between a substrate glass and cover glass of a display device. This approach suffers from the need to create special glass formulations that allow the laser to pass through one glass layer while being absorbed in the second layer. Furthermore, the cover and substrate glasses must have nearly identical coefficients of thermal expansion to prevent residual stresses from being created in the sealed envelope. The special glass formulations add cost and create problems selecting glasses that are compatible with the sealing process, the process for producing the thin-film organic light-emitting devices, and the optical functionality of the completed device. The method suffers also from the need to leave a margin between the seal and the edge of the cover and substrate glasses. Minimizing the distance from the edge of the active area to the edge of the cover and substrate glasses is often a design goal for the device being produced.
Cooper, et al. (U.S. Pat. No. 5,820,435) discloses providing a means for allowing a small gap to exist between the two glass layers of a flat-panel device. This is useful, e.g., for thin-film organic light-emitting devices, where such a gap between the substrate and cover glass is frequently supplied to prevent degradation of the organic materials. However, this approach still suffers the same limitations relating to glass formulation, functional compatibility, and edge margin.
Tracy, et al. (U.S. Pat. No. 5,489,321) introduce a radiation absorbing material between two adjacent glass plates and then subject the material to radiation that has penetrated one of the two glass plates prior to reaching the radiation absorbing material. While this approach probably creates fewer material compatibility problems, especially in terms of allowing both glass layers to be optically transparent, the method still suffers from limiting material choices. It also creates a new problem—producing the energy absorbing material in the right shape and thickness to match the spacing requirements of the completed device. Since the desired gap is typically on the order of 10-20 microns, producing a wire or other form of radiation absorbing material in this thickness or diameter is quite challenging. Furthermore, the bond width and the spacing between the glass plates can limit the volume of energy absorbing material, and potentially the ultimate bond strength.
Li, et al. (US2003/0066311A1) describe a method for sealing a glass substrate and a glass cap by employing a glass frit in a bonding region between the glass substrate and glass cap. This approach eliminates the need to make one of the glass cover or glass cap be produced from the energy absorbing material, but retains the short-coming that the radiation used to heat the frit bonding material still must pass through one of the glass substrate or glass cap. This method also introduces a new problem in that the frit is produced from glass and a binder. The binder must be baked out prior to assembling the device, introducing a new step. In addition, organic thin-film light-emitting devices are very sensitive to contamination, and even after bake-out, the frit will contain small amounts of binder that will be liberated from the frit and trapped in the sealed envelope, risking damage to the encapsulated device.
All of the processes described above are a form of welding, in which the substrate and cover are fused, either directly to one another, or each is fused to a third component, which is typically referred to as the “filler” in welding. Welding contrasts with soldering, in which a material (the solder) wets the surfaces of the cover and substrate and forms a mechanical junction through this wetting, without melting the cover or substrate. Soldering can be utilized in a style similar to that of the frit, in which the solder is placed in a bonding region between the substrate and cover and is heated by any of a variety of means including radiation and induction. Unlike the welding processes described, soldering can be applied from the edges of the glasses to be sealed and heated directly. The solder is then drawn into the gap by capillary action. The main drawback of soldering methods is the cost of solders compatible with glasses, which tends to be high, and the difficulty of controlling the wicking of the solder between the cover and substrate.
There is a need therefore for an improved sealing method which provides a high quality seal without unduly limiting the choice of materials for the cover and substrate.